Conventionally, as a speaker for use in water, there has been, for example, the one described in Japanese Patent No. 3057039 (Japanese Patent Laid-Open Publication No. HEI 11-113082). This speaker, which is to be used by a diver in water, has a cylindrical watertight casing whose fore end surface serves as a diaphragm, a voice coil provided projectingly at the center of the inner surface of this diaphragm, a columnar portion put oppositely in this voice coil and a magnetic circuit section that is provided with a permanent magnet and fixed on an elastic support plate, the elastic support plate peripherally fixed on a front chamber wall of the watertight casing, a control board that is provided with a control section, a digital signal generator section, a changeover section, a D-to-A converter, a power amplifier and so on and is fixed to the lower surface of the magnetic circuit section, and a battery and a switch housed in a rear chamber of the watertight casing partitioned by a partition plate provided with through holes.
If the switch is turned on to make a selection with the changeover section, then a combination signal of the corresponding specific frequencies is inputted from the digital signal generator section to the D-to-A converter, and an analog signal obtained through the conversion is amplified and thereafter inputted to the speaker, consequently producing a sound like “peep-poh peep-poh” or “pah-peep pah-peep” expressing specific information in water.
With regard to the above-mentioned speaker, the diaphragm provided with the projecting voice coil is elastically deformed by a water pressure so as to be pressed inward in accordance with an increase in water depth, and the pressure inside the front chamber, which is partitioned by this diaphragm and the elastic support plate, increases. However, the elastic support plate is also elastically deformed so as to be pressed toward the rear chamber side. Consequently, the magnetic circuit section on the elastic plate is also pressed inward, and the engagement between the voice coil and the magnetic circuit section required for the normal driving of the diaphragm is maintained. That is, the speaker can produce a satisfactory sound even when it receives a water pressure at a water depth of not smaller than 10 m.
However, the aforementioned conventional speaker for diver use has a disadvantage that it is comparatively large and, if downsized, air in the casing generates a distortion due to the vibrations of the diaphragm.
Moreover, there is a disadvantage that, when the elastic support plate that is supporting the magnetic circuit section is reduced in thickness in accordance with the downsizing, the air in the casing repeats compression and expansion when the diaphragm produces a low-pitched sound of a great sound pressure, and the voice coil of the diaphragm becomes unable to smoothly move with respect to the magnetic circuit section, causing unstable operation of the diaphragm.